The reason is because you have a dynamic IP address and when your Internet
connection first comes up, IP Masquerade doesn't know its IP address. There is
a solution to this. In your /etc/rc.d/rc.firewall-* ruleset, add the following:

# Dynamic IP users:
#
# If you get your IP address dynamically from SLIP, PPP, or DHCP, enable this
# following option. This enables dynamic-ip address hacking in IP MASQ, making
# the life with Diald and similar programs much easier.
#
echo "1" > /proc/sys/net/ipv4/ip_dynaddr